The present invention relates to high-intensity discharge lamps and high-intensity discharge lamp operating apparatuses. In particular, the present invention relates to high-intensity discharge lamps such as metal halide lamps, high pressure mercury lamps, and high pressure sodium lamps that are provided with a trigger wire.
Conventionally, high-intensity discharge lamps such as metal halide lamps have been used for general illumination or spot illumination. In recent years, they are also widely used as a light source of OHPs and liquid crystal projectors.
A metal halide lamp includes, for example, an arc tube (luminous bulb) made of quartz glass and a pair of electrodes spaced apart with a predetermined distance in the arc tube, and mercury and a metal halide are enclosed as luminous materials in the arc tube. The arc tube is sealed with electrode sealing portions at both ends, and the pair of electrodes are connected to respective external lead wires via metal foils hermetically sealed in the electrode sealing portions. The external lead wires are electrically connected to an operating circuit (driving device) of the lamp. The operating circuit includes a ballast for restricting current flow to less than a predetermined amount during operation and means for applying a high-voltage pulse voltage.
The lighting operation of metal halide lamps is as follows. In order to start lighting operation, first, it is necessary to apply a voltage to the pair of electrodes to cause insulation breakdown to start discharge. A voltage necessary to cause the insulation breakdown is called xe2x80x9cbreakdown voltagexe2x80x9d, and the breakdown voltage is generally a high voltage of several hundreds times higher than the lamp voltage during steady-state operation.
In order to achieve the compactness and the low cost of the operating circuit, it is preferable that the breakdown voltage is as low as possible. As a technique for lowering the breakdown voltage, a configuration where a trigger wire (close conductor) is wound around the outer circumference of the arc tube is known, which is disclosed in Japanese Laid-Open Patent Publication No. 8-69777, for example. FIG. 11 shows a metal halide lamp having this configuration.
The metal halide lamp shown in FIG. 11 has an arc tube 101 and electrode sealing portions 102a and 102b, and a trigger wire 108 having a first end 108a and a second end 108b is spirally wound around the circumference of the arc tube 101. A pair of electrodes 103a and 103b are opposed to each other inside the arc tube 101. The pair of electrodes 103a and 103b are connected to external lead wires 105a and 105b via the metal foils 104a and 104b in electrode sealing portions 102a and 102b. The second end 108b of the trigger wire 108 is connected to the external lead 105b, and the first end 108a of the trigger wire 108 is wound around the end (near the base) of the electrode sealing portion 102a on the side of the arc tube 101. The external lead wires 105a and 105b are electrically connected to an operating circuit (driving device) 111, and the operating circuit 111 is electrically connected to a power supply 107.
This metal halide lamp is provided with the trigger wire 108, so that the breakdown voltage can be reduced. This is caused by the following mechanism. When a high-voltage pulse voltage is applied from the operation circuit 111 connected to the power supply 107 to the electrodes 103a and 103b, the trigger wire 108 has the same electric potential as that of the electrode 103b. As a result, the trigger wire 108 causes an electrical field having a large electric potential gradient to be formed inside the arc tube 101. This electrical field is likely to cause insulation breakdown of Xe gas between the electrodes 103a and 103b and thus the breakdown voltage can be reduced.
However, in the case of the conventional metal halide lamp provided with the trigger wire 108, although the breakdown voltage can be reduced, color change, an increase of the lamp voltage, or devitrification, which is a phenomenon that the transparency is lost by opaqueness of the arc tube, may occur during operation of the lamp. Furthermore, the luminous flux maintenance factor is reduced, or the lamp cannot be turned on, and thus the lifetime of the lamp tends to be reduced.
As a result of the factors causing the above-described phenomena in depth, the inventors of the present invention found that the electrical field generated by the trigger wire 108 is a large factor. For example, in the case where a driving voltage (lamp voltage) of 65V is applied across the electrodes 103a and 103b during operation, the voltage between the trigger wire 108 connected to the external lead wire 105b and the electrode 103a is also 65V. When the distance between the electrodes 103a and 103b is 3.7 mm, the shortest distance between the first end 108a of the trigger wire 108 and the electrode 103a is 1 mm, then the electric potential gradient of the electrical field formed between the electrodes 103a and 103b is 17.6V/mm. On the other hand, the electric potential gradient of the electrical field formed between the electrode 103a and the trigger wire 108 is 65V/mm, which is more than three times higher than 17.6V/mm, in the largest portion.
In the lamp where a high-voltage pulse voltage is applied, the insulation breakdown of Xe gas is caused, and thus discharge is started, the temperature of the inner wall of the arc tube 101 is increased by subsequent discharge. When the temperature of the inner wall of the arc tube 101 is increased, enclosed material such as metal halide enclosed in the arc tube 101 is evaporated and further ionized. The ionized luminous material is affected more by the electrical field between the trigger wire 108 and the electrode 103a than by the electrical field between the electrodes 103a and 103b, and thus attracted more to the trigger wire 108. More in details, in the case where the applied driving voltage is alternating voltage, when the trigger wire 108 is in the negative electric potential, the luminous material such as sodium in the form of positive ions are attracted to the trigger wire 108, and sodium having a small ion radius moves in the quartz glass and leaks out of the arc tube 101. Thus, the amount of the luminous material in the arc tube 101 is reduced, so that the optical characteristics (color temperature, lamp voltage, luminous flux maintenance factor etc.) are significantly changed, which was found by the inventor of the present invention. Furthermore, sodium or the like breaks the amorphous structure of the quartz glass while moving in the quartz glass, and crystallization of the quartz glass (phase transition to cristobalite) occurs, which causes opaqueness or devitrification of the quartz glass.
Therefore, with the foregoing in mind, it is a main object of the present invention to prolong the lamp lifetime of a high-intensity discharge lamp provided with the trigger wire.
A high-intensity discharge lamp of the present invention includes an arc tube including a pair of electrodes opposed to each other therein; and a trigger wire made of a conductive material provided in an outer circumference of the arc tube. The trigger wire is turned to be in a conductive state with one electrode of the pair of electrodes when a start-up voltage is applied across the pair of electrodes, and discharge is started between the pair of electrodes in a state where an electrical field is formed between the trigger wire that is in the conductive state and the other electrode of the pair of electrodes.
In one embodiment of the present invention, a part of the trigger wire is arranged close to an external lead wire in an insulating state, the external lead wire being electrically connected to the one electrode, and when the start-up voltage is applied, insulation breakdown is caused between the part of the trigger wire and the external lead wire to establish the conductive state.
It is preferable that the part of the trigger wire is arranged close to the external lead wire with a distance of less than 3 mm.
In one embodiment of the present invention, the part of the trigger wire is insulated from the external lead wire with air.
In one embodiment of the present invention, an insulating tape is provided between the part of the trigger wire and the external lead wire for insulation.
In one embodiment of the present invention, the high-intensity discharge lamp further includes a pair of sealing portions for sealing ends of the pair of electrodes, the sealing portions extending from the arc tube, wherein the trigger wire is wound around a region between the arc tube and one of the pair of sealing portions that seals an end of the other electrode.
It is preferable that the arc tube is made of quartz glass.
In one embodiment of the present invention, a metal halide is enclosed as a luminous material in the arc tube, and the metal halide includes an alkaline metal halide.
In one embodiment of the present invention, the alkaline metal halide is a sodium halide.
According to another aspect of the present invention, a high-intensity discharge lamp operating apparatus includes a high-intensity discharge lamp including an arc tube including a pair of electrodes opposed to each other therein, and a trigger wire made of a conductive material provided in an outer circumference of the arc tube; an operating circuit for operating the high-intensity discharge lamp. The trigger wire is not electrically connected to either one of the pair of electrodes. The operating circuit includes driving voltage applying means for applying a driving voltage for maintaining discharge after the discharge is started between the pair of electrodes, and start-up voltage applying means for applying a start-up voltage for starting discharge between the pair of electrodes. The trigger wire is electrically connected to the start-up voltage applying means, and each of the pair of electrodes is electrically connected to the driving voltage applying means.
In one embodiment of the present invention, the start-up voltage applying means includes a timer for controlling the start-up voltage applying means so as to apply the start-up voltage to the trigger wire for a predetermined time during start-up.
In one embodiment of the present invention, the start-up voltage applying means includes detecting means for detecting that lamp operation is started by discharge between the pair of electrodes; and controlling means for stopping application of the start-up voltage when the detecting means detects that lamp operation is started.
In one embodiment of the present invention, the detecting means is a voltage detector that detects a change in voltage between the pair of electrodes.
In one embodiment of the present invention, the detecting means is a current detector that detects a change in current flowing between the pair of electrodes.
In one embodiment of the present invention, the high-intensity discharge lamp further includes a pair of sealing portions for sealing ends of the pair of electrodes, the sealing portion extending from the arc tube, and the trigger wire is wound around a region between one of the pair of sealing portions and the arc tube.
It is preferable that the arc tube is made of quartz glass.
In one embodiment of the present invention, the high-intensity discharge lamp is a metal halide lamp in which a metal halide including an alkaline metal halide is enclosed as a luminous material in the arc tube.
In the present invention, when a start-up voltage is applied across the pair of electrodes, the trigger wire is turned to be in a conductive state with respect to one of the electrodes. Therefore, the breakdown voltage can be lowered, and changes in the characteristics of the lamp can be prevented. As a result, the lamp lifetime of the high-intensity discharge lamp provided with the trigger wire can be prolonged. A part of the trigger wire is arranged close to the external lead wire in an insulating state, so that insulation breakdown is caused between the part of the trigger wire and the external lead wire when a start-up voltage is applied. With this simple configuration, the effects of the present invention can be obtained.
Furthermore, in the case where a portion where the trigger wire is wound around is provided in a region between the sealing portion and the arc tube, the intensity of the electrical field generated by the trigger wire can be large. Therefore, with a comparatively low voltage, the high-intensity discharge lamp can be started more reliably. Furthermore, in the case where the arc tube is made of quartz glass, quartz glass has a higher transmittance than that of ceramic materials, and thus a point light source with which a substantial emission region is small can be realized easily. In addition, in the case where the metal halide enclosed in the arc tube includes an alkaline metal halide, a high-intensity discharge lamp (metal halide lamp) having emission characteristics of high-intensity and excellent color rendering properties can be realized easily.
According to the present invention, when a start-up voltage is applied across the pair of electrodes, the trigger wire is turned to be in a conductive state with respect to one of the electrodes. Therefore, the breakdown voltage can be lowered, and changes in the lamp characteristics can be prevented. As a result, the lamp lifetime of the high-intensity discharge lamp provided with the trigger wire can be prolonged.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.